The present invention belongs to the field of ink jet recording in which ink is ejected as ink droplets, and relates more specifically to an ink jet head for ejecting ink as ink droplets and an ink jet recording apparatus using the ink jet head.
As an ink jet recording system in which ink is ejected as ink droplets, there have been known an electrostatic system in which electrostatic force is caused to act on ink to eject the ink as ink droplets, an electrothermal conversion system in which ink droplets are ejected by the pressure of vapor generated due to heat of a heating element, a piezoelectric system in which mechanical pressure pulse is generated by piezoelectric elements to eject ink droplets, and the like.
As the electrostatic ink jet recording system, there is a system in which ink containing charged fine particles is used, and ink ejection is controlled by utilizing electrostatic force through application of a predetermined voltage (drive voltage) to ejection electrodes (drive electrodes) of an ink jet head in correspondence with image data to record an image corresponding to the image data on a recording medium. For example, an ink jet recording apparatus disclosed in JP 10-138493 A is known as an apparatus using such electrostatic ink jet recording method.
FIG. 12 is a schematic structural view of an example of an ink jet head of the electrostatic ink jet recording apparatus disclosed in JP 10-138493 A. In an ink jet head 200 illustrated in FIG. 12, only one ejection portion of the ink jet head disclosed in JP 10-138493 A is conceptually illustrated. The illustrated ink jet head 200 includes a head substrate 202, an ink guide 204, an insulating substrate (ejection port substrate) 206, a control electrode (ejection electrode) 208, a counter electrode 210, a D.C. bias voltage source 212, and a pulse voltage source 214.
The ink guide 204 is disposed on the head substrate 202, and a through hole (ejection port) 216 is bored through the insulating substrate 206 at a position corresponding to the ink guide 204. The ink guide 204 extends through the through hole 216, and a convex tip end portion 204a thereof protrudes above the surface of the isulating substrate 206 on a recording medium P side. The head substrate 202 and the insulating substrate 206 are arranged to have a predetermined gap therebetween to form a flow path 218 of ink Q.
The control electrode 208 is arranged in a ring shape so as to surround the through hole 216 on the surface of the insulating substrate 206 on the recording medium P side for each ejection portion. The control electrode 208 is connected to the pulse voltage source 214 which generates a pulse voltage according to the image data, and the pulse voltage source 214 is grounded through the D.C. bias voltage source 212.
The counter electrode 210 is arranged at a position opposing the tip end portion 204a of the ink guide 204, and is grounded. The recording medium P is disposed on the surface of the counter electrode 210 on the ink guide 204 side. That is, the counter electrode 210 functions as a platen for supporting the recording medium P.
Upon recording, the ink Q containing fine particles (colorant particles) charged to the same polarity as that of the voltage to be applied to the control electrode 208 is circulated by a not shown ink circulation mechanism in a direction from the right side to the left side in the ink flow path 218 in FIG. 12. For example, a high voltage of 1.5 kV is always applied to the control electrode 208 by the D.C. bias voltage source 212. At this time, a part of the ink Q in the ink flow path 218 passes through the through hole 216 in the insulating substrate 206 due to the capillary phenomenon or the like, and is concentrated at the tip end portion 204a of the ink guide 204.
When the pulse voltage source 214 supplies the control electrode 208 biased to 1.5 kV by the bias voltage source 212 with a pulse voltage of, for example, 0V, the voltage of 1.5 kV obtained by superposition of the pulse voltage on the bias voltage is applied to the control electrode 208. In this state, the electric field strength near the tip end portion 204a of the ink guide 204 is relatively low, so that the ink Q containing colorant particles which are concentrated at the tip end portion 204a of the ink guide 204 is not ejected from the tip end portion 204a. 
On the other hand, when the pulse voltage source 214 supplies a pulse voltage of, for example, 500V, to the control electrode 208 which is biased to 1.5 kV, the voltage of 2 kV obtained by superposition of the pulse voltage on the bias voltage is applied to the control electrode 208. Consequently, the ink Q containing colorant particles which are concentrated at the tip end portion 204a of the ink guide 204 flies as ink droplets R from the tip end portion 204a due to electrostatic force, and is attracted to the grounded counter electrode 210 to adhere to the recording medium P, thereby forming dots of colorant particles.
In this way, recording is performed with dots of colorant particles while relatively moving the ink jet head 200 and the recording medium P supported on the counter electrode 210, thereby recording an image corresponding to the image data on the recording medium P.
In the recording apparatus which uses the ink jet recording system in which ink droplets are ejected from the ejection port (through hole) 216, specially, the electrostatic ink jet recording system, responsivity in ejecting ink droplets can be improved by maintaining a meniscus formed at the ejection port 216 during ink ejection large in height.
A meniscus formed at the ejection port 216 can be maintained large in height by making the opening area of the ejection port 216 larger.
However, when the opening area of the ejection port 216 is made larger, ink flow path resistance at the ejection port 216 is reduced, which leads to a problem that ejection of ink droplets is not stopped even when an ejection signal is stopped, i.e., even when the control electrode 208 transfers from the state where a voltage of 500V is applied from the pulse voltage source 214 to the state where a voltage of 0V is applied from the pulse voltage source 214. That is, the ink ejection cutoff property (ink is not ejected after the end of a drive voltage application) is deteriorated (impaired). Deterioration of the ink ejection cutoff property may cause an error in ejection of ink droplets or the like, thereby raising a problem in that ejection of ink droplets cannot be stably controlled.